1. Field of the Invention
The present invention relates to a physical channel assignment method and a transmitter, and more particularly, is preferably applicable to a cellular radiocommunication system such as a portable telephone system.
2. Description of the Related Art
In the cellular radiocommunication system, an area for providing a communication service has been divided into cells with desired size. A base station has been provided as a fixed station in each of the cells and a communication terminal unit as a mobile station has been designed to perform a radiocommunication with the base station whose communication state seems to be most desirable.
There have been proposed various types of systems as a communication system between the communication terminal unit and the base station. A representative communication system is called a frequency hopping scheme. With this frequency hopping scheme, assuming that, for instance, eight frequency channels f0 to f7 are prepared for communication from the base station to the communication terminal unit, as shown in FIG. 1, the frequency channels used at the time of communication are sequentially changed in terms of time. For example, as shown in FIG. 1, the frequency channels employed are sequentially changed in such a manner that a frequency channel f3 is used in an interval (called a time slot T0, hereinafter) from time t0 to time t1, a frequency channel f1 is used in an interval of time t1 to time t2 (namely, a time slot T1), a frequency channel f5 is used in an interval of time t2 to time t3 (that is to say, a time slot T2) and a frequency channel 2 is used in an interval of time t3 to time t4 (namely, a time slot T3). Since the frequency channels are changed as described above in terms of time, even if an interference wave exists in a certain frequency channel, the influence of an interference only on the same channel can be reduced, because that the frequency channel is not always used.
In such a frequency hopping scheme, a frequency channel determined by a physical parameter such as frequency is looked up as a physical channel. A channel formed by successively combining the frequency channels in terms of time, in other words, a channel formed by the frequency channels f3, f1, f5, f2, . . . in an example shown in FIG. 1, is called a logical channel. In this case, if there are, for instance, eight frequency channels, the number of frequency channels available for each time slot may be eight. Therefore, when the frequency channels are combined together in terms of time, eight logical channels may be also formed. Consequently, even in accordance with the frequency hopping system, if there exist eight frequency channels, the eight logical channels can be formed and eight communications can be carried out simultaneously.
Here, it is assumed that the eight frequency channels f0 to f7 are prepared for communication from the base station to the communication terminal unit. Further, as shown in FIG. 2, it is assumed that eight communication terminal units MS0 to MS7 are present in the service area of an arbitrary base station BS1 and the base station BS1 performs a communication with the eight communication terminal units MS0 to MS7 by using logical channels #0 to #7 formed based on the frequency hopping scheme.
Under these conditions, in the conventional base station, the frequency channels f0 to f7 are respectively assigned to the eight logical channels #0 to #7 in accordance with a method described below.
That is to say, the base station has an offset value V0 peculiar to each of the communication terminal units. The frequency channels of the channel numbers fN obtained as a result of a remainder computation as shown in the following Equation are assigned to the respective logical channels #0 to #7 based on the offset value V0 and a random number value VR changing with time generated in a prescribed random number generator.
fN=(V0+VR)mod8xe2x80x83xe2x80x83(1)
Where, mod indicates a remainder computation. Further specifically, as shown in FIG. 3, it is assumed that the offset values V0 assigned to the communication terminal units MS0 to MS7 are respectively 0, 1, 2, 3, 4, 5, 6 and 7, and the random number values VR are 0, 3, 5, 7, 2, 1, . . . , the frequency channels f0, f3, f5, f7, f2, f1, . . . are assigned to the logical channel #0 used for the communication with the communication terminal unit MS0 based on the above described Equation (1). The frequency channels f1, f4, f6, f0, f3, f2, . . . are assigned to the logical channels #1 used for the communication with the communication terminal unit MS1.
The frequency channels f0 to f7 are assigned to the logical channels #0 to #7 in accordance with the above method, hence the frequency channels can be assigned to the logical channels #0 to #7 so that the same frequency channels are not employed at the same time.
In the above mentioned conventional channel assignment, however, although the different frequency channels f0 to f7 are respectively assigned to the logical channels #0 to #7 at the same time, there exists an inconvenience that the same logical channels are always assigned to the adjacent frequency channels. For instance, when the channel assignment shown in FIG. 3 is represented again by providing the frequency channels on the abscissa axis, this channel assignment can be changed to one as illustrated in FIG. 4. As can be seen from FIG. 4, the logical channels #0 and #2 are always assigned to the frequency channels adjacent to the frequency channel #1 to which the logical channel #1 is assigned. In such a way, if the same logical channels are always located at adjacent positions on a frequency axis, they always undergo an interference wave of the same level (this is called an adjacent channel interference wave) from the adjacent logical channels. As a result, there arises a problem that an interference with adjacent channels is always fixed.
As mentioned above, when the interference with adjacent channels is fixed, this causes a serious problem particularly when transmitted power is controlled. Usually, in the cellular radiocommunication system, parties of communication mutually monitor the received power of a signal sent from one party to the other party and inform him of the monitored result so that the transmitted power is controlled. Accordingly, in the cellular radiocommunication system, the communication can be always performed with minimum transmitted power as required. When the transmitted power is controlled in such a manner, the transmitted power differs every communication (namely, for each of logical channels).
The power state of the logical channels at prescribed time under the control of the transmitted power is illustrated in FIG. 5. As shown in FIG. 5, when the power of the logical channel #1 is low and the power of the logical channels #0 and #2 located at the adjacent positions thereto in terms of frequency is high, the adjacent channel interference waves to the logical channel #1 from the logical channels #0 and #2 are increased because the power of the logical channels #0 and #2 is high under this state. As a result, there arises a risk that a communication cannot be performed through the logical channel #1 because a signal component sent originally from the logical channel #1 is buried in the adjacent channel interference waves. This phenomenon is not generated only at a prescribed time and may be generated at all timings when the same logical channels are always located at the adjacent positions as mentioned above.
According to the conventional channel assignment method as described, since the same logical channels are always located at the adjacent positions, the adjacent channel interference wave is fixed. Consequently, in the worst case, an inconvenience may occur that it is impossible to perform a communication.
In view of the foregoing, an object of this invention is to provide a physical channel assignment method capable of preventing the same logical channels from being always located at the adjacent positions and of averaging an adjacent channel interference and a transmitter for transmitting a signal through logical channels assigned with the method.
The foregoing object and other objects of the invention have been achieved by the provision of a physical channel assignment method upon forming logical channels based on a frequency hopping scheme by combining sequentially physical channels composed of frequency channels in terms of time; the method comprising the steps of: generating first pseudo-noise codes having a prescribed cycle; using the values of the first pseudo-noise codes as the channel numbers of the physical channels and assigning the physical channels respectively to the first logical channel; obtaining second pseudo-noise codes whose phases are shifted from those of the first pseudo-noise codes; and using the values of the second pseudo-noise codes as the channel numbers of the physical channels and assigning the physical channels respectively to the second logical channel.
As described above, the values of the first pseudo-noise codes are used as the channel numbers of the physical channels and the physical channels are respectively assigned to the first logical channel; second pseudo-noise codes whose phases are shifted from those of the first pseudo-noise codes are obtained and the values of the second pseudo-noise codes are used as the channel numbers of the physical channels and the physical channels are respectively assigned to the second logical channel. Thus, the first and second logical channels can be prevented from being always located at the adjacent positions to each other on a frequency axis. Since the first and second logical channels can be prevented from being always adjacent to each other, the same adjacent channel interference can be prevented from being always supplied from the same logical channel, and the adjacent channel interference can be averaged.
Further, according to another aspect of the present invention, there is provided a physical channel assignment method upon forming logical channels based on a frequency hopping mode by combining sequentially physical channels composed of frequency channels in terms of time, the method comprising the steps of previously providing a channel assignment table on which an interference with adjacent channels is uniformly distributed; and using values looked up from the channel assignment table as the channel numbers of the physical channels and assigning the physical channels respectively to the plurality of logical channels.
As described above, the channel assignment table on which an interference with adjacent channels is uniformly distributed is previously prepared and values looked up from the channel assignment table are used as the channel numbers of the physical channels and the physical channels are assigned respectively to the plurality of logical channels. Thus, the same logical channels can be prevented from being always located at the adjacent positions to each other on a frequency axis with a relatively simple configuration, so that the adjacent channel interference can be averaged.
The nature, principle and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts are designated by like reference numerals or characters.